Some variant of Tegmark 4 seems intuitively obvious to me. Indeed it's hard for me to see how any other model fits, once I realized that there really is Nothing Special About Us. The only universe-sets that make sense to me are the empty set and the "all possible universes" set. (with the exact criteria for possibility unknown)
I say this not because it answers the questions raised, but because it's been knocking around in my mind for years without having anyone I could say it to that would get the point.
Some interesting stuff about our conceptions of the world might fall apart if you adopt the mathematical universe. If you think that the entirety of mathematical structures exists in the same way, than it is hard to think what happens when you decide to do good to someone with the entire structure. The whole thing just "is there". Your decision could be thought of as a computational process that takes place in many different subsets. But the exact opposite decision still takes place where it takes place. Then you get something complicated in whic...
There are at least ten different conceptions of how the World can be made of many worlds.
But are those just definitional disputes? Or are they separate claims that can be evaluated. If they are distinct, in virtue of what are they distinct. Finally, do we have good grounds to care (morally) about those fine distinctions?
Max Tegmark's taxonomy is well known here.
Brian Greene's is less, and has 9, instead of four, kinds of multiverse, I'll risk conflating the Tegmark ones that are superclasses of these, feel free to correct me:
I don't understand branes well enough (or at all) to classify the others. The holographic one seems compatible with a multitude, if not all, previous ones.
Besides all those there is David Lewis's Possible Worlds in which all possible worlds exist (in whichever sense the word exist can be significantly applied, if any). For Lewis, when we call our World the Actual World, we think we mean the only one that is there, but what we mean is "the one to which we happen to belong". Notice it is distinct from the Mathematical/Ultimate in that there may be properties of non-mathematical kind.
So Actuallewis= Our world and Actualmost everyone else=Those that obtain, exist, or are real.
The trouble with existence, or reality, is that it is hard to pin down what it is pointing at. Eliezer writes:
and elsewhere
Now another interesting way of looking at existence or reality is
Reality=I should care about what takes place there
It is interesting because it is what is residually left after you abandon the all too stringent standard of "causally connected to me", which would leave few or none of the above, and cut the party short.
So Existenceyud and Existencemoral-concern are very different. Reality-fluid, or Measure, in quantum universes is also different, and sometimes described by some as the quantity of existence. Notice though that the Measure is always a ratio - say these universes here are 30% of the successors of that universe, the other 70% are those other ones - not an absolute quantity.
Which of the 10 kinds of multiverses, besides our own, have Existenceyud Existencemoral-concern and which can be split up in reality-fluid ratios?
That is left as an exercise, since I am very confused by the whole thing...